Hemolysis of Streptococci- Types and Examples with Images
Streptococcus is a genus of gram-positive bacteria that are spherical or ovoid in shape and grow in pairs or chains. They are non-motile, non-spore forming, and mostly facultatively anaerobic. Streptococci are a diverse group of bacteria that can colonize different sites of the human body and cause various diseases ranging from mild infections to severe or chronic conditions.
One of the main characteristics that has been used to classify streptococci is their ability to lyse red blood cells (RBCs) on blood agar plates. This phenomenon is called hemolysis and it is caused by enzymes (hemolysins) released by the bacteria. Hemolysis can be observed as a zone of clearing or discoloration around the bacterial colonies on blood agar.
There are three types of hemolysis: alpha, beta, and gamma.
Hemolysis is the process of breaking down red blood cells (RBCs) and releasing their contents into the surrounding medium. Hemolysis can be caused by various factors, such as toxins, enzymes, antibodies, or physical stress. Hemolysis can have different effects on the blood and the body, depending on the extent and the location of the RBC destruction.
Hemolysis can be classified into two main types: intravascular and extravascular. Intravascular hemolysis occurs when RBCs are lysed within the blood vessels, resulting in the release of hemoglobin and other substances into the plasma. This can cause hemoglobinemia (hemoglobin in the blood), hemoglobinuria (hemoglobin in the urine), and jaundice (yellowing of the skin and eyes due to bilirubin accumulation). Intravascular hemolysis can also lead to anemia (low RBC count), kidney damage, and shock.
Extravascular hemolysis occurs when RBCs are removed from the circulation by macrophages in the spleen, liver, or bone marrow. The macrophages phagocytose (engulf) the RBCs and break them down into their components. The iron from the hemoglobin is recycled and used to make new RBCs, while the heme is converted into bilirubin and excreted in the bile. Extravascular hemolysis can also cause anemia, jaundice, and splenomegaly (enlarged spleen).
Hemolysis can also be classified based on its appearance on blood agar plates. Blood agar is a type of culture medium that contains sheep or horse blood and supports the growth of many bacteria. Some bacteria can produce substances called hemolysins that can lyse RBCs in the blood agar and cause different patterns of hemolysis. There are three types of hemolysis that can be observed on blood agar plates: alpha, beta, and gamma.
Beta-hemolytic streptococci are a group of bacteria that cause complete lysis of red blood cells on blood agar plates, producing a clear zone around the colonies. They produce two types of hemolysins, O and S, that are responsible for the hemolytic activity. Beta-hemolytic streptococci are classified into different serological groups based on their cell wall antigens, such as A, B, C, D, F and G. Some of these groups are important human pathogens that cause a variety of infections.
Group A Streptococcus (GAS)
Group A Streptococcus (GAS), also known as Streptococcus pyogenes, is the most common and virulent beta-hemolytic streptococcus. It causes a wide range of infections, such as pharyngitis, scarlet fever, impetigo, cellulitis, erysipelas, necrotizing fasciitis, rheumatic fever and post-streptococcal glomerulonephritis. GAS can also cause invasive infections, such as bacteremia, septic arthritis and toxic shock syndrome. GAS has several virulence factors that help it evade the host immune system and damage the tissues, such as M protein, hyaluronic acid capsule, streptolysins O and S, streptokinase, streptodornase and exotoxins.
Group B Streptococcus (GBS)
Group B Streptococcus (GBS), also known as Streptococcus agalactiae, is a weakly beta-hemolytic streptococcus that can cause serious infections in newborns and pregnant women. It is the leading cause of neonatal sepsis and meningitis in developed countries. GBS can also cause urinary tract infections, endometritis, chorioamnionitis and postpartum bacteremia in pregnant women. GBS can also infect adults with underlying conditions, such as diabetes mellitus, liver disease and malignancy. GBS has several virulence factors that help it adhere to and invade the host cells, such as capsular polysaccharides, surface proteins, hemolysins and hyaluronidase.
Other Beta-hemolytic Streptococci
Other beta-hemolytic streptococci include groups C, F and G streptococci that are usually found in animals but can occasionally cause human infections. They can cause pharyngitis, tonsillitis, skin and soft tissue infections and bacteremia. They can also cause invasive infections in immunocompromised hosts or those with underlying diseases. Group C streptococci include Streptococcus dysgalactiae subsp. equisimilis and Streptococcus anginosus group. Group F streptococci include Streptococcus milleri group. Group G streptococci include Streptococcus canis and Streptococcus dysgalactiae subsp. dysgalactiae.
Alpha-hemolytic streptococci are gram-positive, spherical, and non-motile organisms whose cells are organized in pairs or short chains. They are facultative anaerobes and form a characteristic alpha-hemolytic zone on blood agar plates. Alpha hemolysis is a partial or smaller zone of hemolysis that has the characteristic narrow grass-green zone that is observed around the colonies on the blood agar. The organism causes oxidization of iron in hemoglobin molecules within red blood cells to produce methemoglobin. There is partial hemolysis of the hemoglobin leaving behind the greenish coloration around the colonies on the agar medium.
Alpha-hemolytic streptococci are commonly called viridans streptococci because of their greenish discoloration on blood agar. They are a heterogeneous group of streptococci that do not express Lancefield antigens and are usually part of the normal flora of the oral cavity, respiratory tract, gastrointestinal tract, and genitourinary tract. They are usually considered as low-virulence organisms, but they can cause opportunistic infections in immunocompromised patients or when they enter the bloodstream through trauma, surgery, or dental procedures.
Some examples of alpha-hemolytic streptococci are:
- Streptococcus pneumoniae: This is the most important and clinically significant alpha-hemolytic streptococcus. It is also known as pneumococcus because it is the main cause of community-acquired pneumonia in adults and children. It can also cause other infections such as otitis media, sinusitis, meningitis, bacteremia, and endocarditis. It is distinguished from other viridans streptococci by its lancet-shaped appearance, its sensitivity to optochin, and its ability to be lysed by bile.
- Streptococcus viridans: This is a collective term for several species of alpha-hemolytic streptococci that are part of the normal oral flora and are involved in dental plaque formation. They include S. mutans, S. sanguinis, S. mitis, S. salivarius, S. oralis, S. anginosus, and others. They can cause dental caries, periodontal disease, and subacute bacterial endocarditis (especially in patients with pre-existing heart valve defects). They can also cause abscesses, septicemia, meningitis, and osteomyelitis in rare cases.
- Streptococcus intermedius group: This is a subgroup of viridans streptococci that includes S. intermedius, S. constellatus, and S. anginosus. They are found in the oral cavity, upper respiratory tract, gastrointestinal tract, and urogenital tract. They are associated with abscess formation in various sites such as the brain, liver, lung, and peritoneum.
Gamma-hemolytic streptococci are gram-positive bacteria that do not cause hemolysis of red blood cells on blood agar plates. They are also called non-hemolytic streptococci because they do not produce any hemolysins that can lyse the erythrocytes. Gamma-hemolytic streptococci are usually commensal organisms that colonize the gastrointestinal tract, genitourinary tract, and skin of humans and animals. However, some species can cause opportunistic infections in immunocompromised hosts or when they enter sterile sites such as the bloodstream, heart valves, or central nervous system.
One of the most important groups of gamma-hemolytic streptococci is the enterococci, which were formerly classified as group D streptococci. Enterococci are facultative anaerobes that can grow in a wide range of environmental conditions, such as high salt concentration, low pH, high temperature, and presence of bile. They are also resistant to many antibiotics, which makes them difficult to treat. Enterococci are responsible for various infections such as urinary tract infections, endocarditis, bacteremia, wound infections, and intra-abdominal infections. The most common species of enterococci that cause human infections are Enterococcus faecalis and Enterococcus faecium.
Another group of gamma-hemolytic streptococci is the nutritionally variant streptococci (NVS), which require pyridoxal or vitamin B6 for growth. They are also called Abiotrophia or Granulicatella species. NVS are part of the normal flora of the oral cavity and upper respiratory tract, but they can also cause endocarditis, bacteremia, meningitis, and other infections. NVS are difficult to culture and identify because they grow poorly on conventional media and often appear as satellite colonies around other bacteria that provide them with pyridoxal.
Other gamma-hemolytic streptococci that are less frequently isolated from human infections include Streptococcus bovis, Streptococcus equinus, Streptococcus gallolyticus, Streptococcus salivarius, Streptococcus thermophilus, and Streptococcus anginosus group. These bacteria have different ecological niches and pathogenic potentials, but they generally do not cause hemolysis on blood agar plates.
This article was based on the following sources of information:
- Tille P.M (2014). Bailey and Scott’s diagnostic microbiology, Thirteen editions, Mosby, Inc., an affiliate of Elsevier Inc., 3251 Riverport Lane, St. Louis, Missouri 63043
- Buxton R (2005). Blood agar plates and hemolysis protocols. American Society for Microbiology.
These sources were selected for their relevance, accuracy, and credibility. They provide a comprehensive overview of the genus Streptococcus and its classification based on hemolytic characteristics. They also explain the concept of hemolysis and its types, and describe the examples of beta-hemolytic, alpha-hemolytic, and gamma-hemolytic streptococci with images.
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